Image capturing apparatus and method of controlling the same

ABSTRACT

An image capturing apparatus comprising an image sensor including a plurality of pixels each including a plurality of photoelectric conversion units, and a first holding unit and a second holding unit configured to store signals output from the plurality of pixels and a driving unit configured to perform first write processing for writing first signals supplied from a first number of photoelectric conversion elements of each pixel in the first holding unit and second write processing for writing second signals supplied from a second number of photoelectric conversion units of each pixel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image capturing apparatus and amethod of controlling the same.

2. Description of the Related Art

Image sensors used in image capturing apparatuses such as digital stillcameras or digital video cameras are highly multi-functionalized. Forexample, some image sensors are known as sensors each having a focusdetection function. Japanese Patent Laid-Open No. 2001-124984 disclosesa technique for allowing focus detection of a phase difference detectionmethod using output signals from two photodiodes which are arrangedtogether with one microlens in one pixel of the image sensor and receivelight beams passing through different pupil areas of an imaging lens.When the output signals from the two photodiodes in a single pixel areadded, an image output can be obtained. With this arrangement, thesignals must be obtained from the two photodiodes for each pixel. Ascompared with a conventional arrangement in which one photodiode isarranged for one pixel, the readout time is undesirably doubled.However, the readout time needs to be shortened to achieve readoutoperation at a predetermined frame rate.

The present invention provides an image capturing apparatus capable ofshortening the signal readout time in the image capturing apparatususing an image sensor capable of performing focus detection of a phasedifference detection method in consideration of the above problem.

SUMMARY OF THE INVENTION

The first aspect of the present invention provides an image capturingapparatus comprising an image sensor including a plurality of pixelseach including a plurality of photoelectric conversion units, and afirst holding unit and a second holding unit configured to store signalsoutput from the plurality of pixels and a driving unit configured toperform first write processing for writing first signals supplied from afirst number of photoelectric conversion elements of each pixel in thefirst holding unit and second write processing for writing secondsignals supplied from a second number of photoelectric conversion unitsof each pixel. The second number is larger than the first number, anddriving the image sensor so as to set a period of the first writeprocessing shorter than a period of the second write processing.

The second aspect of the present invention provides a method ofcontrolling an image capturing apparatus including an image sensorincluding a plurality of pixels each including a plurality ofphotoelectric conversion units, and a first holding unit and a secondholding unit configured to store signals output from the plurality ofpixels. The method comprises a first write processing step of writingfirst signals supplied from a first number of photoelectric conversionelements of each pixel in the first holding unit and a second writeprocessing step of writing second signals supplied from a second numberof photoelectric conversion units of each pixel. The second number beinglarger than the first number and a period of the first write processingstep is set shorter than a period of the second write processing step.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view in which a light beam exiting from the exitpupil of a imaging lens of an image capturing apparatus is incident on aunit pixel;

FIG. 2 is an overall block diagram of the image capturing apparatus;

FIG. 3 is an overall view of an image sensor according to the firstembodiment;

FIG. 4 is a circuit diagram of the unit pixel of the image sensoraccording to the first embodiment;

FIG. 5 is a circuit diagram showing a readout circuit of the imagesensor according to the first embodiment; and

FIG. 6 is a driving timing chart of the image sensor according to thefirst embodiment.

DESCRIPTION OF THE EMBODIMENTS

An exemplary embodiment of the present invention will be described withreference to the accompanying drawings.

First Embodiment

The principle of implementing focus detection of a phase differencedetection method in a general image sensor for capturing an object willbe described first. FIG. 1 is a schematic view showing a state in whicha light beam exiting from the exit pupil of an imaging lens is incidenton one unit pixel of the image sensor. A unit pixel 100 includes a firstphotodiode 101A and a second photodiode 101B. A color filter 302 and amicrolens 303 are arranged in correspondence with the unit pixel 100.

The center of a light beam exiting from the exit pupil 304 of theimaging lens is defined as an optical axis 305 with respect to the pixelhaving the microlens 303. The light passing through the exit pupil 304is incident on the unit pixel 100 using the optical axis 305 as thecenter. Pupil areas 306 and 307 are part of exit pupil areas of theimaging lens. As shown in FIG. 1, the light beam passing through thepupil area 306 is received by the photodiode 101A via the microlens 303.The light beam passing through the pupil area 307 is received by thephotodiode 101B via the microlens 303. In this manner, since thephotodiodes 101A and 101B respectively receive the light beams from thedifferent pupil areas 306 and 307 of the exit pupil 304 of the imaginglens, focus detection based on the phase difference between the signalsfrom the photodiodes 101A and 101B is possible.

Assume that the signal obtained from the photodiode 101A is an A imagesignal, and the signal obtained from the photodiode 101B is a B imagesignal. An (A+B) image signal obtained as the sum of the A and B imagesignals can be used as a captured image signal of the unit pixel 100.

FIG. 2 is a block diagram showing an example of the arrangement of thefunctions of the image capturing apparatus using the image sensor ofthis embodiment. The optical system of the image capturing apparatusincludes an imaging lens 1110 for forming the optical image of an objecton an image sensor 1101. A lens driving circuit 1109 performs zoomcontrol, focus control, aperture control, and the like. The plurality ofunit pixels 100 are arranged in a matrix on the image sensor 1101. Anobject image formed on the image sensor 1101 is output from the imagesensor 1101 as an electrical image signal. A signal processing circuit1103 performs various kinds of correction for the image signal outputfrom the image sensor 1101 and compresses the image signal. The signalprocessing circuit 1103 may generate the B image signal as thedifference signal between the A image signal and (A+B) image signalobtained by the image sensor 1101. A timing generating circuit 1102outputs timing signals for driving the image sensor 1101. An overallcontrol computing circuit 1104 performs various kinds of computation andcontrols the overall operation of the image capturing apparatusincluding the operation of the image sensor 1101. The overall controlcomputing circuit 1104 also performs the focus detection operation of aphase difference detection method using the A and B image signals. Theimage data output from the signal processing circuit 1103 is temporarilystored in a memory circuit 1105. A display circuit 1106 displays variouskinds of information and captured images. A recording circuit 1107records or reads out image data. The recording circuit 1107 is a circuitfor performs read and write accesses for a detachable recording mediumsuch as a semiconductor memory. An operation circuit 1108 includes inputdevices represented by switches, buttons, and a touch panel. Theoperation circuit receives a user instruction to the image capturingapparatus.

The arrangement example of the image sensor 1101 will be described withreference to FIGS. 3 to 5. FIG. 3 is a view showing the overallarrangement example of the image sensor 1101. The image sensor 1101includes a pixel area 1, vertical scanning circuit 2, readout circuit 3,horizontal scanning circuit 4, and output amplifier 5. The plurality ofunit pixels 100 are arranged in a matrix in the image area 1. For thedescriptive convenience, an array of 16 (=4×4) unit pixels isillustrated. In practice, several million unit pixels are arranged in amatrix. As described with reference to FIG. 1, each unit pixel 100includes the first photodiode 101A and the second photodiode 101B. Inthis embodiment, the vertical scanning circuit 2 selects each row ofpixels of the pixel area 1 and sends a driving signal to the pixels ofthe selected row. The readout circuit 3 includes a column readoutcircuit for each column, amplifies the output signal from the unit pixel100, and samples and holds the output signal. The horizontal scanningcircuit 4 outputs a signal for sequentially outputting, to the outputamplifier 5, signals sampled and held by the readout circuit 3. Theoutput amplifier 5 outputs the signal from the readout circuit 3 to thesignal processing circuit 1103 in accordance with the operation of thehorizontal scanning circuit 4. The vertical scanning circuit 2, thereadout circuit 3, and the horizontal scanning circuit 4 are driven bythe timing signals from the timing generating circuit 1102.

FIG. 4 is a circuit diagram showing the arrangement example of the unitpixel 100. A first transfer switch 102A and a second transfer switch102B are connected to the first photodiode 101A and the secondphotodiode 101B, respectively. The outputs of the first and secondtransfer switches 102A and 102B are connected to an amplifier 104 via afloating diffusion region 103. A reset switch 105 is connected to thefloating diffusion region 103, and a selection switch 106 is connectedto the amplifier 104.

The photodiodes 101A and 101B function as photoelectric conversion unitsfor receiving the light beams passing through the same microlens andgenerating signal charges corresponding to the received light amounts.The transfer switches 102A and 102B function as transfer units fortransferring the charges generated by the photodiodes 101A and 101B tothe common floating diffusion region 103. The transfer switches 102A and102B are respectively controlled by transfer pulse signals PTXA and PTXBfrom the vertical scanning circuit 2. The floating diffusion region 103functions as a charge-voltage converter for temporarily holding thecharges transferred from the photodiodes 101A and 101B and convertingthe held charges into a voltage signal. The amplifier 104 includes asource follower MOS transistor, amplifies the voltage signal convertedby the floating diffusion region 103, and outputs the amplified signalas a pixel signal. The reset switch 105 is controlled by a reset pulsesignal PRES from the vertical scanning circuit 2 and resets thepotential of the floating diffusion region 103 to a reference potentialVDD 108. The selection switch 106 is controlled by a vertical selectionpulse signal PSEL from the vertical scanning circuit 2 and outputs thevoltage signal amplified by the amplifier 104 as the pixel signal to avertical output line 107.

FIG. 5 is a circuit diagram showing the arrangement example of a columnreadout circuit 200 included in the readout circuit 3 in FIG. 3. Thereadout circuit 3 includes the column readout circuit 200 for eachcolumn, and the arrangements of the respective columns are common toeach other. Only one column is illustrated. An operation amplifier 203amplifies the signal voltage on the vertical output line 107. Theoperational amplifier 203 receives a reference voltage VREF. A clampcapacitor C0204 is connected to the input of the operational amplifier203, and a feedback capacitor Cf205 is connected to the input and outputof the operational amplifier 203. A switch 206 for short-circuiting thetwo terminals of the feedback capacitor Cf205 is connected to the twoterminals of the feedback capacitor Cf205. The switch 206 is controlledby a PC0R signal from the readout circuit 3. A constant current source201 is connected to the vertical output line 107.

A CTS_A+B 207, CTS_A 208, and CTN 209 are capacitors for holding thesignal voltage from the operational amplifier 203. Switches 210, 211,and 212 are switches to control write to the CTS_A+B 207, CTS_A 208, andCTN 209, respectively. The switch 210 is controlled by the PTS_A+Bsignal from the readout circuit 3; the switch 211, by the PTS_A signalfrom the readout circuit 3; and the switch 212, by the PTN signal fromthe readout circuit 3. A CTS₂ 219 and CTN₂ 220 are capacitors forholding the signal voltages from the CTS_A+B 207, CTS_A 208, and CTN209. Switches 216, 217, and 218 control write to the CTS₂ 219 and CTN₂220. The switch 216 writes the signal of the CTS_A+B 207 in the CTS₂ 219and is controlled by a PTS2_A+B signal from the readout circuit 3. Theswitch 217 writes the signal of the CTS_A 208 in the CTS₂ 219 and iscontrolled by a PTS2_A signal from the readout circuit 3. The switch 218writes the signal of the CTN 209 in the CTN₂ 220 and is controlled by aPTN2 signal from the readout circuit 3.

Voltage follower circuits 213, 214, and 215 serving as buffers arearranged immediately before the CTS₂ 219 and CTN₂ 220. The voltagefollower circuits 213, 214, and 215 transfer potentials equal to thepotentials accumulated in the CTS_A+B 207, CTS_A 208, and CTN 209 to theCTS₂ 219 and CTN₂ 220 without capacitive division.

A switch (not shown) for short-circuiting the two terminals of eachcapacitor as in the feedback capacitor Cf 205 is arranged for each ofthe CTS_A+B 207, CTS_A 208, CTN 209, CTS₂ 219, and CTN₂ 220. By turningon each switch for each readout operation for each row, the CTS_A+B 207,CTS_A 208, CTN 209, CTS₂ 219, and CTN₂ 220 are reset.

Switches 221 and 222 are controlled by a PH signal from the horizontalscanning circuit 4. The signal written in the CTS₂ 219 is output to theoutput amplifier 5 via a common output line 223. The signal written inthe CTN₂ 220 is output to the output amplifier 5 via the common outputline 224. The write operation of signals to the CTS_A+B 207, CTS_A 208,and CTN 209 and the signal readout operation from the CTS₂ 219 and CTN₂220 by horizontal scanning are performed parallel.

FIG. 6 is a timing chart for implementing the method of driving theimage sensor 1101 according to this embodiment. FIG. 6 shows the drivingtimings when the vertical scanning circuit 2 selects a given row. Attime t1, a horizontal synchronization signal SYNC rises, and at the sametime, the vertical selection pulse signal PSEL of the selected rowchanges from L level to H level. This makes it possible to turn on theselection switch 106 (FIG. 4) of the selected row and outputs the pixelsignal of the selected row to the vertical output line 107.

At time t2, the reset pulse signal PRES changes from L level to H levelto turn on the reset switch 105 and reset the potential of the floatingdiffusion region 103 to the power supply voltage VDD. At time t3, thereset pulse signal PRES changes from H level to L level to turn off thereset switch 105 and cancel the reset state of the floating diffusionregion 103. At this time, the potential of the floating diffusion region103 is read out as a reset signal level to the vertical output line 107via the amplifier 104 and input to the column readout circuit 200. Inthe column readout circuit 200, the switch 206 (FIG. 5) is set ONbecause the PC0R signal is set at H level. The reset signal level isinput to the clamp capacitor C0204 in a state in which the operationalamplifier 203 buffers the output of the reference voltage VREF.

At time t4, the PC0R signal then changes from H level to L level. Attime t5, the PTN signal changes to H level to turn on the switch 212.The output from the operational amplifier 203 at this time is written inthe capacitor CTN 209 as the reset voltage. Thereafter, at time t6, thePTN signal changes to L level to turn off the switch 212, there bycompleting the write operation of the reset voltage to the CTN 209.

At time t7, the PTS_A signal is set at H level to turn on the switch211, thereby setting a state in which the signal can be written in thecapacitor CTS_A 208. Subsequently, at time t8, the transfer pulse signalPTXA is set at H level to transfer the charges of the photodiode 101A tothe floating diffusion region 103. At time t9, the transfer pulse signalPTXA is set at L level. This operation makes it possible to read out thecharges accumulated in the photodiode 101A to the floating diffusionregion 103. The floating diffusion region 103 converts the charges intoa voltage. The voltage is supplied to the column readout circuit 200 viathe amplifier 104 and the vertical output line 107.

In the column readout circuit 200, the operational amplifier 203amplifies the voltage from the vertical output line 107 at a gaincorresponding to a ratio of the capacitances of the clamp capacitorC0204 and the feedback capacitor Cf205 and outputs the amplifiedvoltage. The signal voltage supplied from the photodiode 101A is writtenin the capacitor CTS_A 208. At time t10, the PTS_A signal changes from Hlevel to L level to turn off the switch 211, thereby completing thewrite operation in the capacitor CTS_A 208. A signal write period (timet7 to time t10) to the capacitor CTS_A 208 is defined as T1.

At time t11, the PTS_A+B signal is set at H level to turn on the switch210, thereby setting a state in which a signal can be written in thecapacitor CTS_A+B 207. Subsequently, at time t12, the transfer pulsesignal PTXA is set at H level again, and at the same time the transferpulse signal PTXB is also set at H level. This operation makes itpossible to simultaneously read out the charges of both the photodiodes101A and 101B to the floating diffusion region 103. At time t14, thetransfer pulse signals PTXA and PTXB are then set at L level. Thereadout charges are converted into a voltage which is then supplied tothe column readout circuit 200. The converted voltage is then amplifiedby the operational amplifier 203. The signal amplified by theoperational amplifier 203 is written in the capacitor CTS_A+B 207. Attime t16, the PTS_A+B signal changes from H level to L level to turn offthe switch 207, thereby completing the write operation in the capacitorCTS_A+B 207. The signal write period for the capacitor CTS_A+B 207, thatis, time t11 to time t16, is defined as T2.

Note that at time t12, the transfer pulse signals PTXA and PTXB aresimultaneously set at H level. However, only the transfer pulse signalPTXB may be set at H level, and the charges of the photodiode 101B maybe transferred to the floating diffusion region 103. In this case, sincethe charges of the photodiode 101A have already been transferred to thefloating diffusion region 103, an output as the sum of the charges ofthe photodiodes 101A and 101B can be obtained. With the above operation,the signal write operations for the capacitors CTS 209, CTS_A 208, andCTS_A+B 207 are completed.

The (A+B) image signal as the sum of the output signals from thephotodiodes 101A and 101B can be obtained from the difference betweenthe signals held in the capacitors CTS_A+B 207 and CTN 209. This (A+B)image signal serves as an image signal. In addition, the A image signalas the output signal from the photodiode 101A is obtained from thedifference between the signals held in the capacitors CTS_A 208 and CTN209. Information of the light beam passing through part of the pupil ofthe imaging lens is obtained from the A image signal, and the A signalis subtracted from the (A+B) image signal to obtain the difference,thereby obtaining the B image signal as the output signal from thephotodiode 101B. Information of a light beam passing through a pupilarea different from that of the image signal can be obtained from the Bimage signal. Therefore, focus information and distance information canbe obtained from the pieces of information of the two light beamsrepresented by the A and B image signals.

It takes a relatively long time to write the signals of the pixel unitin the capacitors CTS_A 208 and CTS_A+B 207 in consideration of thetransmission delay on the vertical output line and the stabilizationperiod of the operational amplifier 203. When the stabilization periodis short, fixed pattern noise such as shading occurs on the signal.There exists a strong demand of reducing the fixed pattern noise in thecaptured image signal ((A+B) image signal) as compared with the focusdetection signals (A and B image signals). In other words, the fixedpattern noise need not be reduced in the focus detection signals ascompared with the captured image signal. In this embodiment, a signal tobe written in the capacitor CTS_A 208 is used for focus detection, and asignal to be written in the capacitor CTS_A+B 207 is used for thecaptured image. The A image signal for focus detection has a voltagelower than that of the (A+B) image signal for the captured image. Forthis reason, the write period T1 for writing a signal in the capacitorCTS_A 208 for the A image signal as the focus detection signal is setshorter than the write period T2 for writing a signal in the capacitorCTS_A+B 207 for the (A+B) image signal as the captured image signal. Forexample, the A signal write period T1 is set as 5.8 μsec, and the A+Bsignal write period T2 is set as 6.3 μsec. That is, the A signal writeperiod T1 is set shorter than the A+B signal write period T2 by 0.5 sec.This makes it possible to shorten the readout time while suppressing thenoise of the captured image.

In order to write the signal in the capacitor CTS_A 208, the timing(time t7) at which the PTS_A signal is set at H level is set earlierthan the timing at which the transfer pulse signal PTXA is set at Hlevel in this embodiment, but may be later than that. In this case, thewrite period T1 for the capacitor CTS_A 208 may be defined as a perioduntil the PTS_A signal is set at L level (time t10) after the transferpulse single PTXA is set at H level (time t8). Similarly, in order towrite a signal in the capacitor CTS_A+B 207, the timing (time t11) atwhich the PTS_A+B signal is set at H level may be set later than thetiming (time t12) at which the transfer pulse signals PTXA and PTXB areset at H level. In this case, the write period T2 for the capacitorCTS_A+B 207 may be defined as a period until the PTS_A+B signal is setat L level (time t16) after the transfer pulse signals PTXA and PTXB areset at H level (time t12).

Referring back to FIG. 6, at time t17, the PC0R signal is set at H levelagain to turn on the switch 206. In the column readout circuit 200, thestate returns to a state in which the operational amplifier 203 buffersthe output of the reference voltage VREF. At time t18, the verticalselection pulse signal PSEL is set at L level to complete the signalreadout operation from the pixels of the selected row. The selected rowis disconnected from the vertical output line. Thereafter, at time t19,the horizontal synchronization signal SYNC set at H level again and thevertical selection pulse signal PSEL of the next row is set at H level,thereby starting selection of the pixels of the next row.

The time returns to time t12. At time t12, the (A+B) image signal iswritten in the capacitor CTS_A+B 207, and at the same time, the PTS2_Asignal and PTN2 signal change from L level to H level, thereby turningon the switches 217 and 218. The signal held in the capacitor CTN 209 iswritten in the capacitor CTN₂ 220 via the voltage follower circuit 215,and the signal held in the capacitor CTS_A 208 is written in thecapacitor CTS₂ 219 via the voltage follower circuit 214. At time t13,the PINS signal and the PTS2_A+B signal simultaneously change from Hlevel to L level, thereby completing the write operations in thecapacitors CTN₂ 220 and CTS₂ 219.

Thereafter, a period between time t13 and time t15, the driving pulse PHof the horizontal scanning circuit 4 sequentially changes in an order ofL level, H level, and L level for each readout circuit of each column.Along with this, the switches 221 and 222 are turned off, on, and offfor each column, thereby performing horizontal scanning. The signalsheld in the capacitors CTS₂ 219 and CTN₂ 220 of each column when theswitches 221 and 222 are turned off, on, and off for each column areread out to the common output lines 223 and 224. The output amplifier221 outputs the signals as the difference voltage of the signals fromthe common output lines 223 and 224. The difference voltage is the Aimage signal. Note that the common output lines 223 and 224 are reset tothe reference potential by a reset switch (not shown) every time thesignals of each column are read out.

In this case, horizontal scanning of the A image signal is performedduring (period T2) the write operation of the (A+B) image signal in thecapacitor CTS_A+B 207. This horizontal scanning period is completeduntil time t16 of completing write operation in the capacitor CTS_A+B207. After the end of the write operation of the (A+B) signal in thecapacitor CTS_A+B 207, write operation and horizontal scanning of the(A+B) image signal in the capacitor CTS₂ 219 are performed. Theseoperations are performed parallelly with the resetting of the floatingdiffusion region 103 for the next row and write operations in thecapacitors CTN 209 and CTS_A 208. At time t17, the PTN2 and PTS2_A+Bsignals simultaneously change from L level to H level to turn on theswitches 216 and 218. The signal held in the capacitor CTN 209 iswritten in the capacitor CTN₂ 220 via the voltage follower circuit 215.In addition, the signal held in the capacitor CTS_A+B 207 is written inthe capacitor CTS₂ 219 via the voltage follower circuit 213. At timet18, the PTN2 and PTS2_A+B signals simultaneously change from H level toL level to complete the write operation.

The driving pulse PH of the horizontal scanning circuit 4 sequentiallychanges in an order of L level, H level, and L level during a period oftime t18 to t20 to perform horizontal scanning of the signals held inthe capacitors CTN₂ 220 and CTS₂ 219 in the example of FIG. 6. Theswitches 221 and 222 of each column are driven by the driving pulse PHin an order of OFF, ON, and OFF. The signals held in the capacitors CTS₂219 and CTN₂ 220 are read out to the common output lines 223 and 224 bythe switches 221 and 222. The output amplifier 221 outputs readoutsignals as the difference voltage between signals of the common outputlines 223 and 224. This difference voltage is the (A+B) image signal.

The horizontal scanning of the (A+B) image signal is performedparallelly with resetting of the floating diffusion region 103 for thenext row and write in the capacitors CTN 209 and CTS_A 208. The parallelprocessing can shorten the processing time. The horizontal scanningperiod can be completed until the next time t10 at which write in thecapacitor CTS_A 208 for the next row ends.

As described above, according to this embodiment, the time required towrite the focus detection signal in the capacitor is made shorter thanthe time required for writing the captured signal in the capacitor,thereby shorting the time required for one-frame read access withoutadversely affecting the captured signal.

This embodiment employs an arrangement including the two photodiodes inthe unit pixel. The number of photodiodes in the unit pixel is notlimited to two and can suffice if a plurality of photodiodes are used.For example, four photodiodes may be arranged in the unit pixel. Signalsof two photoelectric conversion units in the unit pixel may be read outfor focus detection, and the signals of all the photoelectric conversionunits in the unit pixel may be read out for image capturing.

Second Embodiment

An example of applying an image sensor of the present invention to animage capturing apparatus will be described with reference to thecircuit block of FIG. 2. A signal from an overall control computingcircuit 1104 is supplied to a timing generating circuit 1102 inaccordance with an operation from an operation circuit 1108. An objectimage is formed on an image sensor 1101 by an imaging lens 1110 andaperture of an optical system. The timing generating circuit 1102supplies timing signals to the image sensor to control it. The objectimage formed on the image sensor is converted into an electrical signalby the image sensor, and the electrical signal is output from the imagesensor 1101. Note that although the image sensor 1101 can have afunction of converting an analog signal formed on the image sensor intoa digital single, but a signal processing circuit 1103 may perform thisconversion. The image processing circuit 1103 corrects and compresses anoutput signal from the image sensor 1101. The processed image signal istemporarily stored in a memory circuit 1105 under the control of theoverall control and computing circuit 1104 or displayed on a displaycircuit 1106 in accordance with an operation from the operation circuit1108. The overall control computing circuit 1104 transfers the imagesignal from the signal processing circuit 1103 to a recording circuit1107 and stores it in the recording circuit 1107. The recording circuit1107 can be a detachable semiconductor memory.

OTHER EMBODIMENTS

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions recorded on a storage medium (e.g., non-transitorycomputer-readable storage medium) to perform the functions of one ormore of the above-described embodiment(s) of the present invention, andby a method performed by the computer of the system or apparatus by, forexample, reading out and executing the computer executable instructionsfrom the storage medium to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or more ofa central processing unit (CPU), micro processing unit (MPU), or othercircuitry, and may include a network of separate computers or separatecomputer processors. The computer executable instructions may beprovided to the computer, for example, from a network or the storagemedium. The storage medium may include, for example, one or more of ahard disk, a random-access memory (RAM), a read only memory (ROM), astorage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-093051, filed Apr. 25, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image capturing apparatus comprising: an imagesensor including a plurality of pixels each including a plurality ofphotoelectric conversion units, and a first holding unit and a secondholding unit configured to store signals output from the plurality ofpixels; and a driving unit configured to perform first write processingfor writing first signals supplied from a first number of photoelectricconversion elements of each pixel in the first holding unit and secondwrite processing for writing second signals supplied from a secondnumber of photoelectric conversion units of each pixel, the secondnumber being larger than the first number, and driving the image sensorso as to set a period of the first write processing shorter than aperiod of the second write processing.
 2. The apparatus according toclaim 1, wherein each of the plurality of pixels includes acharge-voltage conversion unit shared by the plurality of photoelectricconversion units and a transfer unit configured to transfer chargesgenerated by the plurality of photoelectric conversion units to thecharge-voltage conversion unit.
 3. The apparatus according to claim 1,wherein each of the plurality of pixels includes one microlens.
 4. Theapparatus according to claim 1, wherein each of the plurality of pixelsincludes two photoelectric conversion units, the first signal is asignal supplied from one photoelectric conversion unit of the pixel, andthe second signal is a signal obtained by mixing signals supplied fromthe two photoelectric conversion units.
 5. The apparatus according toclaim 1, wherein the second signal is used for a captured image, and thefirst signal and a difference signal between the second signal and thefirst signal are used for focus detection.
 6. A method of controlling animage capturing apparatus including an image sensor including aplurality of pixels each including a plurality of photoelectricconversion units, and a first holding unit and a second holding unitconfigured to store signals output from the plurality of pixels, themethod comprising: a first write processing step of writing firstsignals supplied from a first number of photoelectric conversionelements of each pixel in the first holding unit; a second writeprocessing step of writing second signals supplied from a second numberof photoelectric conversion units of each pixel, the second number beinglarger than the first number, wherein a period of the first writeprocessing step is set shorter than a period of the second writeprocessing step.
 7. The method according to claim 6, wherein each of theplurality of pixels includes a charge-voltage conversion unit shared bythe plurality of photoelectric conversion units and a transfer unitconfigured to transfer charges generated by the plurality ofphotoelectric conversion units to the charge-voltage conversion unit. 8.The method according to claim 6, wherein each of the plurality of pixelsincludes one microlens.
 9. The method according to claim 6, wherein eachof the plurality of pixels includes two photoelectric conversion units,the first signal is a signal supplied from one photoelectric conversionunit of the pixel, and the second signal is a signal obtained by mixingsignals supplied from the two photoelectric conversion units.
 10. Themethod according to claim 6, wherein the second signal is used for acaptured image, and the first signal and a difference signal between thesecond signal and the first signal are used for focus detection.